Vehicles may be equipped with various exhaust aftertreatment devices to reduce the release of exhaust emissions into the atmosphere. For example, three-way catalysts may reduce levels of various emissions including carbon monoxide and unburnt hydrocarbons while selective catalyst reduction (SCR) systems may be used to reduce levels of NOx. To ensure the aftertreatment devices are functioning optimally, various sensors may be installed upstream and/or downstream of the devices, and feedback from the sensors may be used to determine catalyst conversion efficiency and thereby, degradation in SCR devices.
An example diagnostic approach is shown by Nilsson (WO 2013/152780 A1) where NOx sensor readings are obtained during different diagnostic sequences performed when the vehicle is stationary. One diagnostic sequence includes transitioning from a high NOx to a low NOx output followed by a return to a high NOx output. A second diagnostic sequence includes using a fuel cut to induce low NOx levels while a third diagnostic sequence involves determining catalyst conversion efficiency by delivering high NOx output to a heated catalyst and varying the quantity of injected reductant from zero to a fixed amount. High NOx output is induced by providing a selected fuel injection timing, a high engine speed or applied engine loads. In each sequence, NOx sensor feedback is compared with predetermined thresholds to diagnose SCR catalyst and/or sensor performance.
The inventors herein have identified potential issues with the above approach. Not only are the various processes somewhat complicated, but reliance on stationary testing can be detrimental when the vehicle is not operated in a stationary position sufficiently. Further, the various adjustments to urea dosing as described in '780 can negatively affect emissions in a moving vehicle when performed over longer durations necessary to generate a satisfactory number of readings.
The inventors herein have recognized the above issue and identified an approach to at least partly address the issue. In one example approach, a method for monitoring a SCR catalyst system coupled to an engine in a moving vehicle is provided. The method comprises monitoring SCR conversion efficiency without any adjustments to engine operation when SCR device temperature is below a first threshold and once SCR device temperature is above the first threshold, SCR conversion efficiency is monitored after artificially inducing an increase in feedgas NOx. SCR device degradation is indicated based on conversion efficiency data obtained during both operation conditions.
For example, when SCR device temperature is below a threshold but above light-off temperature, conversion efficiency may be measured based on a relative change in NOx levels from upstream of SCR catalyst to those downstream of SCR catalyst. As such, the system may comprise two sensors to measure said NOx levels: one placed upstream of SCR device and the other placed downstream of SCR device. Additionally, any decline in conversion efficiency may be noted in this mode. Once SCR device temperature is above a threshold and in a functioning range with a higher expected conversion efficiency, feedgas NOx may be temporarily and artificially increased, for e.g. by reducing EGR (exhaust gas recirculation) flow or by advancing fuel injection timing to monitor conversion efficiency. This increase in feedgas NOx may improve signal-to-noise ratio in NOx sensors and may provide a more precise measurement of conversion efficiency. Further, SCR degradation may be confirmed if conversion efficiency in one or both operating modes is below an expected threshold.
In this way, a more accurate diagnosis of SCR catalyst performance can be made based on conversion efficiency data obtained over a range of SCR device operating temperatures and by artificially increasing feedgas NOx levels during high SCR performance to enable more reliable sensor feedback. Moreover, using a reduced EGR flow to raise feedgas NOx levels may offer stable combustion conditions, thereby, improving driveability while increasing feedgas NOx by advancing fuel injection timing can benefit fuel efficiency. In one example, by artificially raising the levels of feedgas NOx only when the SCR catalyst is expected to convert NOx more efficiently, tailpipe emissions can be maintained within acceptable limits. For example, feedgas NOx levels can be increased during highway driving conditions when SCR device is already operating in a peak NOx conversion range. In another example, EGR can be disabled during uphill driving conditions and the increased NOx output can be advantageously used to monitor SCR performance. Thus, SCR performance may be diagnosed during vehicle travel and under different driving situations with minimum intrusion on driveability and emissions.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.